Presynaptic Action Potentials Research Articles

VAMP4 Maintains a Ca2+-Sensitive Pool of Spontaneously Recycling Synaptic Vesicles.

Spontaneous neurotransmitter release is a fundamental property of synapses in which neurotransmitter filled vesicles release their content independent of presynaptic action potentials (APs). Despite their seemingly random nature, these spontaneous fusion events can be regulated by Ca2+ signaling pathways. Here, we probed the mechanisms that maintain Ca2+ sensitivity of spontaneous release events in synapses formed between hippocampal neurons cultured from rats of both sexes. In this setting, we examined the potential role of vesicle-associated membrane protein 4 (VAMP4), a vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein in spontaneous neurotransmission. Our results show that VAMP4 is required for Ca2+-dependent spontaneous excitatory neurotransmission, with a limited role in spontaneous inhibitory neurotransmission. Key residues in VAMP4 that regulate its retrieval as well as functional clathrin-mediated vesicle trafficking were essential for the maintenance of VAMP4-mediated spontaneous release. Moreover, high-frequency stimulation (HFS) that typically triggers asynchronous release and retrieval of VAMP4 from the plasma membrane also augmentsCa2+-sensitive spontaneous release for up to 30 min in a VAMP4-dependent manner. This VAMP4-mediated link between asynchronous and spontaneous excitatory neurotransmission might serve as a presynaptic substrate for synaptic plasticity coupling distinct forms of release.SIGNIFICANCE STATEMENT Spontaneous neurotransmitter release that occurs independent of presynaptic action potentials (APs) shows significant sensitivity to intracellular Ca2+ levels. In this study, we identify the vesicular soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) molecule vesicle-associated membrane protein 4 (VAMP4) as a key component of the machinery that maintains these Ca2+-sensitive fraction of spontaneous release events. Following brief intense activity, VAMP4-dependent synaptic vesicle retrieval supports a pool of vesicles that fuse spontaneously in the long term. We propose that this vesicle trafficking pathway acts to shape spontaneous release and associated signaling based on previous activity history of synapses.

The Frog Motor Nerve Terminal Has Very Brief Action Potentials and Three Electrical Regions Predicted to Differentially Control Transmitter Release.

The action potential (AP) waveform controls the opening of voltage-gated calcium channels and contributes to the driving force for calcium ion flux that triggers neurotransmission at presynaptic nerve terminals. Although the frog neuromuscular junction (NMJ) has long been a model synapse for the study of neurotransmission, its presynaptic AP waveform has never been directly studied, and thus the AP waveform shape and propagation through this long presynaptic nerve terminal are unknown. Using a fast voltage-sensitive dye, we have imaged the AP waveform from the presynaptic terminal of male and female frog NMJs and shown that the AP is very brief in duration and actively propagated along the entire length of the terminal. Furthermore, based on measured AP waveforms at different regions along the length of the nerve terminal, we show that the terminal is divided into three distinct electrical regions: A beginning region immediately after the last node of Ranvier where the AP is broadest, a middle region with a relatively consistent AP duration, and an end region near the tip of nerve terminal branches where the AP is briefer. We hypothesize that these measured changes in the AP waveform along the length of the motor nerve terminal may explain the proximal-distal gradient in transmitter release previously reported at the frog NMJ.SIGNIFICANCE STATEMENT The AP waveform plays an essential role in determining the behavior of neurotransmission at the presynaptic terminal. Although the frog NMJ is a model synapse for the study of synaptic transmission, there are many unknowns centered around the shape and propagation of its presynaptic AP waveform. Here, we demonstrate that the presynaptic terminal of the frog NMJ has a very brief AP waveform and that the motor nerve terminal contains three distinct electrical regions. We propose that the changes in the AP waveform as it propagates along the terminal can explain the proximal-distal gradient in transmitter release seen in electrophysiological studies.

Heterosynaptic Plasticity in Cortical Interneurons.

Persistent alteration of synaptic strength in response to neuronal activity has long been considered a substrate of learning and memory ([Hebb, 1949][1]). Classical Hebbian plasticity results from near-coincidental presynaptic and postsynaptic action potentials (APs), with the precise order of

The General Anesthetic Isoflurane Bilaterally Modulates Neuronal Excitability

SummaryVolatile anesthetics induce hyperactivity during induction while producing anesthesia at higher concentrations. They also bidirectionally modulate many neuronal functions. However, the neuronal mechanism is unclear. The effects of isoflurane on sodium channel currents were analyzed in acute mouse brain slices, including sodium leak (NALCN) currents and voltage-gated sodium channels (Nav) currents. Isoflurane at sub-anesthetic concentrations increased the spontaneous firing rate of CA3 pyramidal neurons, whereas anesthetic concentrations of isoflurane decreased the firing rate. Isoflurane at sub-anesthetic concentrations enhanced NALCN conductance but minimally inhibited Nav currents. Isoflurane at anesthetic concentrations depressed Nav currents and action potential amplitudes. Isoflurane at sub-anesthetic concentrations depolarized resting membrane potential (RMP) of neurons, whereas hyperpolarized the RMP at anesthetic concentrations. Isoflurane at low concentrations induced hyperactivity in vivo, which was diminished in NALCN knockdown mice. In conclusion, enhancement of NALCN by isoflurane contributes to its bidirectional modulation of neuronal excitability and the hyperactivity during induction.

Simplified calcium signaling cascade for synaptic plasticity

We propose a model for synaptic plasticity based on a calcium signaling cascade. The model simplifies the full signaling pathways from a calcium influx to the phosphorylation (potentiation) and dephosphorylation (depression) of glutamate receptors that are gated by fictive C1 and C2 catalysts, respectively. This model is based on tangible chemical reactions, including fictive catalysts, for long-term plasticity rather than the conceptual theories commonplace in various models, such as preset thresholds of calcium concentration. Our simplified model successfully reproduced the experimental synaptic plasticity induced by different protocols such as (i) a synchronous pairing protocol and (ii) correlated presynaptic and postsynaptic action potentials (APs). Further, the ocular dominance plasticity (or the experimental verification of the celebrated Bienenstock—Cooper—Munro theory) was reproduced by two model synapses that compete by means of back-propagating APs (bAPs). The key to this competition is synapse-specific bAPs with reference to bAP-boosting on the physiological grounds.

Target-wide Induction and Synapse Type-Specific Robustness of Presynaptic Homeostasis.

Presynaptic homeostatic plasticity (PHP) is an evolutionarily conserved form of adaptive neuromodulation and is observed at both central and peripheral synapses. In this work, we make several fundamental advances by interrogating the synapse specificity of PHP. We define how PHP remains robust to acute versus long-term neurotransmitter receptor perturbation. We describe a general PHP property that includes global induction and synapse-specific expression mechanisms. Finally, we detail a novel synapse-specific PHP expression mechanism that enables the conversion from short- to long-term PHP expression. If our data can be extended to other systems, including the mammalian central nervous system, they suggest that PHP can be broadly induced and expressed to sustain the function of complex neural circuitry.

Gambierol Potently Increases Evoked Quantal Transmitter Release and Reverses Pre- and Post-Synaptic Blockade at Vertebrate Neuromuscular Junctions

Gambierol is a marine polycyclic ether toxin, first isolated from cultured Gambierdiscus toxicus dinoflagellates collected in French Polynesia. The chemical synthesis of gambierol permitted the analyses of its mode of action which includes the selective inhibition of voltage-gated K+ (KV) channels. In the present study we investigated the action of synthetic gambierol at vertebrate neuromuscular junctions using conventional techniques. Gambierol was studied on neuromuscular junctions in which muscle nicotinic ACh receptors have been blocked with d-tubocurarine (postsynaptic block), or in junctions in which quantal ACh release has been greatly reduced by a low Ca2+–high Mg2+ medium or by botulinum neurotoxin type-A (BoNT/A) (presynaptic block). Results show that nanomolar concentrations of gambierol inhibited the fast K+ current and prolonged the duration of the presynaptic action potential in motor nerve terminals, as revealed by presynaptic focal current recordings, increased stimulus-evoked quantal content in junctions blocked by high Mg2+–low Ca2+ medium, and by BoNT/A, reversed the postsynaptic block produced by d-tubocurarine and increased the transient Ca2+ signals in response to nerve-stimulation (1–10 Hz) in nerve terminals loaded with fluo-3/AM. The results suggest that gambierol, which on equimolar basis is more potent than 3,4-diaminopyridine, can have potential application in pathologies in which it is necessary to antagonize pre- or post-synaptic neuromuscular block, or both.This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Elevated synaptic vesicle release probability in synaptophysin/gyrin family quadruple knockouts.

This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Optical Quantal Analysis.

Understanding the mechanisms by which long-term synaptic plasticity is expressed remains an important objective in neuroscience. From a physiological perspective, the strength of a synapse can be considered a consequence of several parameters including the probability that a presynaptic action potential (AP) evokes the release of neurotransmitter, the mean number of quanta of transmitter released when release is evoked, and the mean amplitude of a postsynaptic response to a single quantum. Various methods have been employed to estimate these quantal parameters from electrophysiological recordings; such “quantal analysis” has been used to support competing accounts of mechanisms of expression of long-term plasticity. Because electrophysiological recordings, even with minimal presynaptic stimulation, can reflect responses arising at multiple synaptic sites, these methods are open to alternative interpretations. By combining intracellular electrical recording with optical detection of transmission at individual synapses, however, it is possible to eliminate such ambiguity. Here, we describe methods for such combined optical and electrical monitoring of synaptic transmission in brain slice preparations and illustrate how quantal analyses thereby obtained permit more definitive conclusions about the physiological changes that underlie long-term synaptic plasticity.

Timing constraints of action potential evoked Ca2+ current and transmitter release at a central nerve terminal

The waveform of presynaptic action potentials (APs) regulates the magnitude of Ca2+ currents (ICa) and neurotransmitter release. However, how APs control the timing of synaptic transmission remains unclear. Using the calyx of Held synapse, we find that Na+ and K+ channels affect the timing by changing the AP waveform. Specifically, the onset of ICa depends on the repolarization but not depolarization rate of APs, being near the end of repolarization phase for narrow APs and advancing to the early repolarization phase for wide APs. Increasing AP amplitude has little effect on the activation but delays the peak time of ICa. Raising extracellular Ca2+ concentration increases the amplitude of ICa yet does not alter their onset timing. Developmental shortening of APs ensures ICa as a tail current and faithful synaptic delay, which is particularly important at the physiological temperature (35 °C) as ICa evoked by broad pseudo-APs can occur in the depolarization phase. The early onset of ICa is more prominent at 35 °C than at 22 °C, likely resulting from a temperature-dependent shift in the activation threshold and accelerated gating kinetics of Ca2+ channels. These results suggest that the timing of Ca2+ influx depends on the AP waveform dictated by voltage-gated channels and temperature.

Pharmacological Dissection of Intrinsic Optical Signal Reveals a Functional Coupling between Synaptic Activity and Astrocytic Volume Transient.

The neuronal activity-dependent change in the manner in which light is absorbed or scattered in brain tissue is called the intrinsic optical signal (IOS), and provides label-free, minimally invasive, and high spatial (~100 µm) resolution imaging for visualizing neuronal activity patterns. IOS imaging in isolated brain slices measured at an infrared wavelength (>700 nm) has recently been attributed to the changes in light scattering and transmittance due to aquaporin-4 (AQP4)-dependent astrocytic swelling. The complexity of functional interactions between neurons and astrocytes, however, has prevented the elucidation of the series of molecular mechanisms leading to the generation of IOS. Here, we pharmacologically dissected the IOS in the acutely prepared brain slices of the stratum radiatum of the hippocampus, induced by 1 s/20 Hz electrical stimulation of Schaffer-collateral pathway with simultaneous measurement of the activity of the neuronal population by field potential recordings. We found that 55% of IOSs peak upon stimulation and originate from postsynaptic AMPA and NMDA receptors. The remaining originated from presynaptic action potentials and vesicle fusion. Mechanistically, the elevated extracellular glutamate and K+ during synaptic transmission were taken up by astrocytes via a glutamate transporter and quinine-sensitive K2P channel, followed by an influx of water via AQP-4. We also found that the decay of IOS is mediated by the DCPIB- and NPPB-sensitive anion channels in astrocytes. Altogether, our results demonstrate that the functional coupling between synaptic activity and astrocytic transient volume change during excitatory synaptic transmission is the major source of IOS.

Presynaptic Diversity Revealed by Ca2+-Permeable AMPA Receptors at the Calyx of Held Synapse.

GluA2-lacking Ca2+-permeable AMPARs (CP-AMPARs) play integral roles in synaptic plasticity and can mediate excitotoxic cellular signaling at glutamatergic synapses. However, the developmental profile of functional CP-AMPARs at the auditory brainstem remains poorly understood. Through a combination of electrophysiological and live-cell Ca2+ imaging from mice of either sex, we show that the synaptic release of glutamate from the calyx of Held nerve terminal activates CP-AMPARs in the principal cells of the medial nucleus of the trapezoid body in the brainstem. This leads to significant Ca2+ influx through these receptors before the onset of hearing at postnatal day 12 (P12). Using a selective open channel blocker of CP-AMPARs, IEM-1460, we estimate that ∼80% of the AMPAR population are permeable to Ca2+ at immature P4-P5 synapses. However, after the onset of hearing, Ca2+ influx through these receptors was greatly reduced. We estimate that CP-AMPARs comprise approximately 40% and 33% of the AMPAR population at P18-P22 and P30-P34, respectively. By quantifying the rate of EPSC block by IEM-1460, we found an increased heterogeneity in glutamate release probability for adult-like calyces (P30-P34). Using tetraethylammonium (TEA), a presynaptic potassium channel blocker, we show that the apparent reduction of CP-AMPARs in more mature synapses is not a consequence of presynaptic action potential (AP) speeding. Finally, through postsynaptic AP recordings, we show that inhibition of CP-AMPARs reduces spike fidelity in juvenile synapses, but not in more mature synapses. We conclude that the expression of functional CP-AMPARs declines over early postnatal development in the calyx of Held synapse.SIGNIFICANCE STATEMENT The calyx of Held synapse is pivotal to the circuitry that computes sound localization. Postsynaptic Ca2+ influx via AMPARs may be critical for signaling the maturation of this brainstem synapse. The GluA4 subunit may dominate the AMPAR complex at mature synapses because of its fast gating kinetics and large unitary conductance. The expectation is that AMPARs dominated by GluA4 subunits should be highly Ca2+ permeable. However, we find that Ca2+-permeable AMPAR expression declines during postnatal development. Using the rate of EPSC block by IEM-1460, an open channel blocker of Ca2+-permeable AMPARs, we propose a novel method to determine glutamate release probability and uncover an increased heterogeneity in release probability for more mature calyces of Held nerve terminals.

The "Memory" Effect in a Chain of Biochemical Reactions with a Positive Feedback is Enhanced by Substrate Saturation Described by Michaelis-Menten Kinetics.

Information in the brain is stored in a form of an altered synaptic strength between neurons. The long-term potentiation (LTP), a phenomenon when a short-term increase in neural activity is transformed into a long-lasting increase in strengths of synaptic connectivity, provides an experimental substrate of memory. Using reaction-diffusion equations, we established an LTP model, describing the dynamics of glutamate (Glu), calcium (Ca2+) and nitric oxide (NO) in response to the stimulus-a presynaptic action potential. NO can diffuse to the presynaptic terminal and facilitate the Glu release forming a positive feedback loop. Therefore, the LTP can be considered as a chain of biochemical reactions with a positive feedback loop. In this study, we investigated numerically the role of interactions in a chain of biochemical reactions with a positive feedback on the bistable behavior or memory. We conclude that the positive feedback system with the linear interaction between substances does not exhibit a bistable behavior. However, introduction of substrate saturation described by Michaelis-Menten kinetics for NO decay can lead to an increase in synaptic strength lasting for dozens or even hundreds of seconds. Our finding extends a possible role of NO in LTP: a short high intensity stimulus is "memorized" as a long-lasting elevation of NO concentration.

Molecular Mechanisms of Short-Term Plasticity: Role of Synapsin Phosphorylation in Augmentation and Potentiation of Spontaneous Glutamate Release.

We used genetic and pharmacological approaches to identify the signaling pathways involved in augmentation and potentiation, two forms of activity dependent, short-term synaptic plasticity that enhance neurotransmitter release. Trains of presynaptic action potentials produced a robust increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs). Following the end of the stimulus, mEPSC frequency followed a bi-exponential decay back to basal levels. The time constants of decay identified these two exponential components as the decay of augmentation and potentiation, respectively. Augmentation increased mEPSC frequency by 9.3-fold, while potentiation increased mEPSC frequency by 2.4-fold. In synapsin triple-knockout (TKO) neurons, augmentation was reduced by 83% and potentiation was reduced by 74%, suggesting that synapsins are key signaling elements in both forms of plasticity. To examine the synapsin isoforms involved, we expressed individual synapsin isoforms in TKO neurons. While synapsin IIIa rescued both augmentation and potentiation, none of the other synapsin isoforms produced statistically significant amounts of rescue. To determine the involvement of protein kinases in these two forms of short-term plasticity, we examined the effects of inhibitors of protein kinases A (PKA) and C (PKC). While inhibition of PKC had little effect, PKA inhibition reduced augmentation by 76% and potentiation by 60%. Further, elevation of intracellular cAMP concentration, by either forskolin or IBMX, greatly increased mEPSC frequency and occluded the amount of augmentation and potentiation evoked by electrical stimulation. Finally, mutating a PKA phosphorylation site to non-phosphorylatable alanine largely abolished the ability of synapsin IIIa to rescue both augmentation and potentiation. Together, these results indicate that PKA activation is required for both augmentation and potentiation of spontaneous neurotransmitter release and that PKA-mediated phosphorylation of synapsin IIIa underlies both forms of presynaptic short-term plasticity.

Sevoflurane inhibits presynaptic calcium influx without affecting presynaptic action potentials in hippocampal CA1 region.

Although diverse effects of volatile anesthetics have been investigated in various studies, the mechanisms of action of such anesthetics, especially sevoflurane, remain elusive. In contrast to their potent modulation of inhibitory synaptic transmission there is little information about their effects on excitatory transmission in the brain. In this study, we examined the effect of sevoflurane on the excitatory synaptic transmission at CA1 synapses in hippocampal slices of mice. Sevoflurane at 5% was mixed with 95% O2 and 5% CO2 and bubbled in artificial cerebral spinal fluid (0.69 mM). Extracellular recordings of the field excitatory postsynaptic potential (fEPSP) and presynaptic fiber volley (FV) were made at physiological temperature. In addition, fluorescent measurements of presynaptic Ca2+ transients were performed while simultaneously recording fEPSP. Application of sevoflurane reduced the amplitude of fEPSP (45 ± 8%, n = 5). This effect was accompanied by concurrent enhancement of the paired-pulse facilitation of fEPSP (127 ± 5%, n = 12), suggesting a possible presynaptic site of action of sevoflurane. The amplitude of FV was not significantly affected (102 ± 5%, n = 5). In contrast, fluorescent measurements revealed that presynaptic Ca2+ influx was suppressed by sevoflurane (69 ± 5%, n = 7), as was simultaneously recorded fEPSP (44 ± 5%, n = 7). Our results suggest that sevoflurane potently suppresses excitatory synaptic transmission via inhibition of presynaptic Ca2+ influx without affecting presynaptic action potentials.

Parvalbumin-Interneuron Output Synapses Show Spike-Timing-Dependent Plasticity that Contributes to Auditory Map Remodeling

Parvalbumin (PV)-expressing interneurons mediate fast inhibition of principal neurons in many brain areas; however, long-term plasticity at PV-interneuron output synapses has been less well studied. In the auditory cortex, thalamic inputs drive reliably timed action potentials (APs) in principal neurons and PV-interneurons. Using paired recordings in the input layer of the mouse auditory cortex, we found a marked spike-timing-dependent plasticity (STDP) at PV-interneuron output synapses. Long-term potentiation of inhibition (iLTP) is observed upon postsynaptic (principal neuron) then presynaptic (PV-interneuron) AP firing. The opposite AP order causes GABAB-mediated long-term depression of inhibition (iLTD), which is developmentally converted to iLTP in an experience-dependent manner. Genetic deletion of GABAB receptors in principal neurons suppressed iLTD and produced deficits in auditory map remodeling. Output synapses of PV-interneurons thus show marked STDP, and one limb of this plasticity, GABAB-dependent iLTD, is a candidate mechanism for disinhibition during auditory critical period plasticity.

Copine-6 Binds to SNAREs and Selectively Suppresses Spontaneous Neurotransmission

Recent studies suggest that spontaneous and action potential-evoked neurotransmitter release processes are independently regulated. However, the mechanisms that uncouple the two forms of neurotransmission remain unclear. In cultured mouse and rat neurons, we show that the two C2 domain-containing protein copine-6 is localized to presynaptic terminals and binds to synaptobrevin2 as well as other SNARE proteins in a Ca2+-dependent manner. Ca2+-dependent interaction of copine-6 with synaptobrevin2 selectively suppresses spontaneous neurotransmission in a reaction that requires the tandem tryptophan residues at the C-terminal region of synaptobrevin2. Accordingly, copine-6 loss of function augmented presynaptic Ca2+ elevation-mediated neurotransmitter release. Intracellular Ca2+ chelation, on the other hand, occluded copine-6-mediated suppression of release. We also evaluated the molecular specificity of the copine-6-dependent regulation of spontaneous release and found that overexpression of copine-6 did not suppress spontaneous release in synaptobrevin2-deficient neurons. Together, these results suggest that copine-6 acts as a specific Ca2+-dependent suppressor of spontaneous neurotransmission.SIGNIFICANCE STATEMENT Synaptic transmission occurs both in response to presynaptic action potentials and spontaneously, in the absence of stimulation. Currently, much more is understood about the mechanisms underlying action potential-evoked neurotransmission compared with spontaneous release. However, recent studies have shown selective modulation of spontaneous neurotransmission process by several neuromodulators, suggesting specific molecular regulation of spontaneous release. In this study, we identify copine-6 as a specific regulator of spontaneous neurotransmission. By both gain-of-function and loss-of-function experiments, we show that copine-6 functions as a Ca2+-dependent suppressor of spontaneous release. These results further elucidate the mechanisms underlying differential regulation of evoked and spontaneous neurotransmitter release.

β-Secretase BACE1 Promotes Surface Expression and Function of Kv3.4 at Hippocampal Mossy Fiber Synapses.

The β-secretase β-site APP-cleaving enzyme 1 (BACE1) is deemed a major culprit in Alzheimer's disease, but accumulating evidence indicates that there is more to the enzyme than driving the amyloidogenic processing of the amyloid precursor protein. For example, BACE1 has emerged as an important regulator of neuronal activity through proteolytic and, most unexpectedly, also through nonproteolytic interactions with several ion channels. Here, we identify and characterize the voltage-gated K+ channel 3.4 (Kv3.4) as a new and functionally relevant interaction partner of BACE1. Kv3.4 gives rise to A-type current with fast activating and inactivating kinetics and serves to repolarize the presynaptic action potential. We found that BACE1 and Kv3.4 are highly enriched and remarkably colocalized in hippocampal mossy fibers (MFs). In BACE1-/- mice of either sex, Kv3.4 surface expression was significantly reduced in the hippocampus and, in synaptic fractions thereof, Kv3.4 was specifically diminished, whereas protein levels of other presynaptic K+ channels such as KCa1.1 and KCa2.3 remained unchanged. The apparent loss of presynaptic Kv3.4 affected the strength of excitatory transmission at the MF-CA3 synapse in hippocampal slices of BACE1-/- mice when probed with the Kv3 channel blocker BDS-I. The effect of BACE1 on Kv3.4 expression and function should be bidirectional, as predicted from a heterologous expression system, in which BACE1 cotransfection produced a concomitant upregulation of Kv3.4 surface level and current based on a physical interaction between the two proteins. Our data show that, by targeting Kv3.4 to presynaptic sites, BACE1 endows the terminal with a powerful means to regulate the strength of transmitter release.SIGNIFICANCE STATEMENT The β-secretase β-site APP-cleaving enzyme 1 (BACE1) is infamous for its crucial role in the pathogenesis of Alzheimer's disease, but its physiological functions in the intact nervous system are only gradually being unveiled. Here, we extend previous work implicating BACE1 in the expression and function of voltage-gated Na+ and K+ channels. Specifically, we characterize voltage-gated K+ channel 3.4 (Kv3.4), a presynaptic K+ channel required for action potential repolarization, as a novel interaction partner of BACE1 at the mossy fiber (MF)-CA3 synapse of the hippocampus. BACE1 promotes surface expression of Kv3.4 at MF terminals, most likely by physically associating with the channel protein in a nonenzymatic fashion. We advance the BACE1-Kv3.4 interaction as a mechanism to strengthen the temporal control over transmitter release from MF terminals.

Protein Kinase C Enhances Electrical Synaptic Transmission by Acting on Junctional and Postsynaptic Ca2+ Currents.

By synchronizing neuronal activity, electrical transmission influences the coordination, pattern, and/or frequency of firing. In the hemaphroditic marine-snail, Aplysia calfornica, the neuroendocrine bag cell neurons use electrical synapses to synchronize a 30 min afterdischarge of action potentials for the release of reproductive hormone. During the afterdischarge, protein kinase C (PKC) is activated, although its impact on bag cell neuron electrical transmission is unknown. This was investigated here by monitoring electrical synapses between paired cultured bag cell neurons using dual whole-cell recording. Voltage clamp revealed a largely voltage-independent junctional current, which was enhanced by treating with a PKC activator, PMA, before recording. We also examined the transfer of presynaptic action potential-like waveforms (generated in voltage clamp) to the postsynaptic cell (measured in current clamp). For control pairs, the presynaptic spike-like waveforms mainly evoked electrotonic potentials; however, when PKC was triggered, these stimuli consistently produced postsynaptic action potentials. To assess whether this involved changes to postsynaptic responsiveness, single bag cell neurons were injected with junctional-like current mimicking that evoked by a presynaptic action potential. Unlike control neurons, which were less likely to spike, cells in PMA always fired action potentials to the junctional-like current. Furthermore, PKC activation increased a postsynaptic voltage-gated Ca2+ current, which was recruited even by modest depolarization associated with an electrotonic potential. Whereas PKC inhibits gap junctions in most systems, bag cell neurons are rather unique, as the kinase potentiates the electrical synapse; in turn, this synergizes with augmented postsynaptic Ca2+ current to promote synchronous firing.SIGNIFICANCE STATEMENT Electrical coupling is a fundamental form of communication. For the bag cell neurons of Aplysia, electrical synapses coordinate a prolonged burst of action potentials known as the afterdischarge. We looked at how protein kinase C, which is upregulated with the afterdischarge, influences information transfer across the synapse. The kinase activation increased junctional current, a remarkable finding given that this enzyme is largely considered inhibitory for gap junctions. There was also an augmentation in the ability of a presynaptic neuron to provoke postsynaptic action potentials. This increased excitability was, in part, due to enhanced postsynaptic voltage-dependent Ca2+ current. Thus, protein kinase C improves the fidelity of electrotonic transmission and promotes synchronous firing by modulating both junctional and membrane conductances.

Lambert-Eaton myasthenic syndrome: mouse passive-transfer model illuminates disease pathology and facilitates testing therapeutic leads.

Lambert-Eaton myasthenic syndrome (LEMS) is an autoimmune disorder caused by antibodies directed against the voltage-gated calcium channels that provide the calcium ion flux that triggers acetylcholine release at the neuromuscular junction. To study the pathophysiology of LEMS and test candidate therapeutic strategies, a passive-transfer animal model has been developed in mice, which can be created by daily intraperitoneal injections of LEMS patient serum or IgG into mice for 2-4 weeks. Results from studies of the mouse neuromuscular junction have revealed that each synapse has hundreds of transmitter release sites but that the probability for release at each one is likely to be low. LEMS further reduces this low probability such that transmission is no longer effective at triggering a muscle contraction. The LEMS-mediated attack reduces the number of presynaptic calcium channels, disorganizes transmitter release sites, and results in the homeostatic upregulation of other calcium channel types. Symptomatic treatment is focused on increasing the probability of release from dysfunctional release sites. Current treatment uses the potassium channel blocker 3,4-diaminopyridine (DAP) to broaden the presynaptic action potential, providing more time for calcium channels to open. Current research is focused on testing new calcium channel gating modifiers that work synergistically with DAP.